faqs.org - Internet FAQ Archives

RFC 1998

Or Display the document by number

Network Working Group                                            E. Chen
Request for Comments: 1998                                           MCI
Category: Informational                                         T. Bates
                                                           cisco Systems
                                                             August 1996

             An Application of the BGP Community Attribute
                         in Multi-home Routing

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.


   This document presents an application of the BGP community attribute
   [2] in simplifying the implementation and configuration of routing
   policies in the multi-provider Internet. It shows how the community
   based configuration can be used to replace the AS-based customization
   of the BGP "LOCAL_PREF" attribute, a common method used today.  Not
   only does the technique presented simplifies configuration and
   management at the provider level, it also represents a paradigm shift
   in that it gives the potential for the customer to control its own
   routing policy with respect to its service provider, as well as
   providing the ability for policy configuration to be done at a prefix
   based granularity rather than the more common AS based granularity.

1. Introduction

   In the multi-provider Internet, it is common for a service subscriber
   (i.e., customer) to have more than one service provider, or to have
   arrangements for redundant connectivity to the global connected
   Internet. As discussed in [3], routing strategies in these cases
   usually require coordination between the service subscriber and its
   providers, which typically leads to customization of router
   configurations (e.g., BGP "LOCAL_PREF") not only by the subscriber,
   but also by its providers.  Due to the large number of customers a
   provider serves, customization of router configurations at the
   provider level may present management and scalability problems.

   This document presents an application of the BGP community attribute
   in simplifying the implementation of routing strategies in the
   multi-provider Internet.  More specifically, the technique presented
   uses a community-based, rather than the common AS-based,

   configuration of the BGP "LOCAL_PREF". It essentially removes the
   need for customized configuration of the BGP "LOCAL_PREF" attribute
   at the provider level while maintaining the same level of routing
   functionality and flexibility.

   It also represents a paradigm shift in that it gives the potential
   for the customer to control its own routing policy with respect to
   its service provider, as well as providing the ability for policy
   configuration to be done at a prefix based granularity rather than
   the more common AS based granularity in use today.

2. AS-based Configuration and its Drawbacks

   As discussed in [3], in today's multi-provider Internet, customized
   configuration of the BGP "LOCAL_PREF" attribute is often required to
   implement common routing strategies such as load-sharing or backup.
   There are two main reasons:

     o Lack of available implementations and deployment of routing
       software that supports the "Destination Preference Attribute"
       (DPA) as specified in [4].

       DPA allows one to specify a globally transitive preference so
       that return traffic favors certain path. As discussed in [3],
       the attribute will be very useful in influencing route selection
       for routes with identical "LOCAL_PREF" and equal AS-path length.

     o In the multi-provider Internet, it is common for a provider
       to assign higher BGP "LOCAL_PREF" values for routes from its
       customers than from other service providers. This practice
       provides some degree of protection for its customer routes,
       and it facilitates implementation of certain routing
       strategies.  It, however, also complicates other routing
       implementations such as backup arrangement, thus, requiring
       customized "LOCAL_PREF" configuration.

   Figure 1 shows a typical case of a backup arrangement in the multi-
   provider Internet. In Figure 1, AS1 and AS2 are both providers, and
   AS3 and AS4 are customers of AS1 and AS2, respectively. AS3 has
   entered a bilateral agreement with AS4 to provide backup to each
   other.  That is, AS3 would use its direct link to AS4 to reach only
   AS4 in the normal circumstance, and for transit in the case of a
   failure between AS3 and AS1.  To realize this routing agreement, AS3
   requests that its provider AS1 adjust its BGP "LOCAL_PREF"
   configuration so that AS1 reaches AS4 via AS2.

                          +------+      +------+
                          | AS1  |------| AS2  |
                          +------+      +------+
                             |             |
                          +------+      +------+
                          | AS3  |------|  AS4 |
                          +------+      +------+

                     Figure 1: Typical Backup Scenario

   Primarily due to scalability and management concerns, most providers
   only perform "LOCAL_PREF" customization based on ASs, not on IP
   prefixes.  If IP prefix-based "LOCAL_PREF" configuration is needed, a
   technique known as as the BGP AS-path manipulation can be used.
   However, it is currently only available in certain vendor's products.

   There are several drawbacks with the the practice of AS-based BGP
   "LOCAL_PREF" configuration at the provider level:

      o The implementation tends to less efficient due to the process
        of coordination and configuration.  More importantly, the
        process needs to be repeated each time a change (e.g., adding
        a new AS) occurs.

      o The AS-based customization complicates router configuration
        and increases complexity of network operation. It has become
        a serious scalability issue for providers.

      o It can not implement prefix-based configuration without the
        AS-path manipulation (i.e., using fake AS).

      o Keeping configuration up-to-date is some times problematic.

3. How the BGP Community Attribute Can Help

3.1 Overview of the Community Attribute

   The BGP community path attribute is an optional transitive attribute
   of variable length [1,2]. The attribute consists of a set of four
   octet values, each of which specify a community.  The community
   attribute values are encoded using an AS number in the first two
   octets, with the remaining two octets defined by the AS. As defined
   in [2], a community is a group of destinations (i.e. prefixes) that
   share some common attribute.  Each destination can belong to multiple
   communities.  All prefixes with the community attribute belong to the
   communities listed in the attribute.

   The BGP community  allows one to group a set of prefixes and perform
   routing decisions based on the identity of the group.

   The well-known communities NO_EXPORT (0xFFFFFF01) and NO_ADVERTISE
   (0xFFFFFF02) are intuitive,  and can be used for optimizing routing
   and for improving route aggregation.

3.2 Community-based Configuration

   With the BGP community attribute [2], a provider can now use
   community-based, rather than AS-based, configuration of BGP
   "LOCAL_PREF".  The provider first needs to coordinate with its
   customers a set of communities to be mapped to certain BGP
   "LOCAL_PREF" values.  The provider can then apply a uniform BGP
   configuration to all its customers that would capture routes with the
   community values, and set up the appropriate BGP "LOCAL_PREF" values
   accordingly.  A customer that requires customization in its provider
   BGP "LOCAL_PREF" configuration can simply send the appropriate
   community values in its routing announcements.

   The major advantages of using this technique include:

      o The customer has full control in the process, which makes a
        lot of sense as the customer is in a position to have better
        understanding about its own topology and routing policy

      o The effect of route-based customization in BGP "LOCAL_PREF"
        configuration by providers can now be achieved, thus, removing
        the need of AS-Path manipulation in certain cases.

      o It addresses the scalability issue facing providers as it
        distributes the configuration work to the customer that
        requires customization.

4. A Real-World Implementation Example

   MCI currently makes heavy use of the BGP "LOCAL_PREF" attribute value
   as part of its routing policy configuration process.  Different BGP
   "LOCAL_PREF" values are assigned for routes from different sources.
   Table 1 details these values:

                  |        Category         | LOCAL_PREF |
                  |Customer Routes          |        100 |
                  |Customer backup Routes   |         90 |
                  |Other ISP Routes         |         80 |
                  |Customer-Provided backup |         70 |

                    Table 1: Defined LOCAL_PREF Values


       o The value '100' is the default value used within our network

       o In most cases, the MED attribute set by a customer is
         sufficient for customer backup routes (e.g., T1 backs up T3).
         However, in certain cases configuration of "LOCAL_PREF" will
         still be necessary until the BGP DPA attribute is available.

   To make use of the BGP community attribute, several community values
   (MCI's AS number: 3561 = 0x0DE9) have been defined that can be used
   by customers to tag routes so that the appropriate "LOCAL_PREF"
   values are configured. Table 2 lists the appropriate community
   attribute values (and the mappings of community to LOCAL_PREF):

                    |     community       | LOCAL_PREF |
                    |3561:70 (0x0DE90046) |         70 |
                    |3561:80 (0x0DE90050) |         80 |
                    |3561:90 (0x0DE9005A) |         90 |

                 Table 2: Community to LOCAL_PREF Mapping

   A customer requiring MCI to configure BGP "LOCAL_PREF" values other
   than the default can tag their routes with the defined communities.
   The community values can be configured either based on an AS path
   list or an IP address access list. A cisco systems software specific
   configuration example is given in Appendix A to show how this can be

   A uniform BGP configuration (see Appendix B, again cisco systems
   software specific) is applied by MCI to peers with customers that
   configure the appropriate "LOCAL_PREF" values based on the
   communities received.

   This technique has been tested and is in use with several customers,
   and the response has been very positive. We are in the process of
   migrating all other customized BGP "LOCAL_PREF" configurations to
   this uniform community based configuration approach.

5. References

   [1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
       RFC 1771, March 1995.

   [2] Chandra, R., Traina, P., and T. Li, "BGP Communities
       Attribute", RFC 1997, August 1996.

   [3] Chen, E., and T. Bates, "Current Practice of Implementing
       Symmetric Routing and Load Sharing in the Multi-Provider
       Internet", Work in Progress.

   [4] Chen, E., and T. Bates, "Destination Preference Attribute for
       BGP", Work in Progress.

   [5] Chen, E., and T. Bates, "Application of the BGP Destination
       Preference Attribute in Implementing Symmetric Routing",
       Work in Progress.

   [6] cisco systems, cisco IOS Software Version 10.3 Router Products
       Configuration Guide (Addendum), May 1995.

6. Security Considerations

   Security issues are not discussed in this memo.

7. Acknowledgments

   The authors would specifically like to thank Ravi Chandra, Tony Li
   and Paul Traina of cisco systems for devising and implementing the
   community attribute.

8. Authors' Addresses

   Enke Chen
   2100 Reston Parkway
   Reston, VA 22091

   Phone: +1 703 715 7087
   EMail: enke@mci.net

   Tony Bates
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134

   Phone: +1 408 527 2470
   EMail: tbates@cisco.com


   These appendices list cisco systems software specific configuration
   examples for configuring communities, and for uniform route-map
   definition that sets up the appropriate "LOCAL_PREF" values based on
   the corresponding community values. These examples are given purely
   to show a working example of how the desired effect discussed in this
   document can be achieved. Please refer to [6] for more specific
   information on cisco configuration and syntax.

Appendix A. Community Configuration

   The community values can be configured either based upon an AS path
   list or based an IP address access list. Here is an example that
   includes both cases:

   router bgp xxxx
   neighbor x.x.x.x remote-as 3561
   neighbor x.x.x.x filter-list 20 out
   neighbor x.x.x.x route-map config-community out
   neighbor x.x.x.x send-community
   !!# match all
   ip as-path access-list 1 permit .*
   !!# list of customer ASs
   ip as-path access-list 20 permit ^$
   ip as-path access-list 20 permit ^64700_
   ip as-path access-list 20 deny .*
   !!# AS path based matching, backup for another ISPs customer
   ip as-path access-list 40 permit _64710_
   ip as-path access-list 40 permit _64711_
   ip as-path access-list 40 deny .*
   !!# route-map
   route-map config-community permit 10
   match as-path 40
   set community 0x0DE90046
   route-map config-community permit 20
   match as-path 1

   Note: The community can also be configured based on IP prefixes
   instead of AS numbers.  For example,

   access-list 101 permit ip
   route-map config-community permit 10
   match ip address 101
   set community 0x0DE90046
   route-map config-community permit 20
   match as-path 1

Appendix B. Uniform Route-map Configuration

   Here is the uniform route-map that can be used for all BGP

   !!# routes primary via another ISP
   ip community-list 70 permit 0x0DE90046
   ip community-list 70 deny
   !!# routes also homed to another ISP, but with DPA or
   !!# AS-path length as the tie-breaker
   ip community-list 80 permit 0x0DE90050
   ip community-list 80 deny
   !!# customer backup routes
   ip community-list 90 permit 0x0DE9005A
   ip community-list 90 deny
   !!# the route-map applied to BGP customers
   route-map set-customer-local-pref permit 10
   match community 70
   set local-preference 70
   route-map set-customer-local-pref permit 20
   match community 80
   set local-preference 80
   route-map set-customer-local-pref permit 30
   match community 90
   set local-preference 90
   route-map set-customer-local-pref permit 40
   match as-path 1
   set local-preference 100